In recent years, CMOS image sensors have often been used in digital single-lens reflex cameras and video cameras. An increase in the number of pixels, an increase in image capturing speed, and an increase in ISO speed (an improvement of sensitivity) have been required for such CMOS image sensors.
Pixel size tends to become smaller due to an increase in the number of pixels, and this means that less electric charge can be accumulated in each pixel. Meanwhile, in order to accommodate an increase in ISO speed, a larger gain needs to be applied to the obtained electric charge. Although the original optical signal component is amplified when gain is applied, noise generated by circuits and the like is also amplified, and therefore high ISO speed images have more random noise than low ISO speed images.
Also, one method of realizing high-speed image capturing is multichannelization in which the image sensor is provided with a plurality of output paths, and readout is performed simultaneously for a plurality of pixels. However, since the amount of noise varies depending on the output path, there is the problem that the amount of noise differs for each CH (for each channel).
Below is a description of the configuration of a CMOS image sensor and the cause of noise generation. FIG. 9 shows an overall layout of the CMOS image sensor. As shown in FIG. 9, the CMOS image sensor includes an aperture pixel area (effective pixel area) 903 having aperture pixels (effective pixels), and a vertical optical black area (VOB, first reference pixel area) 902 and a horizontal optical black area (HOB, second reference pixel area) 901 that have shielded pixels (reference pixels). The HOB 901 is provided adjacent to the head (on the left side) of the aperture pixel area 903 in the horizontal direction, and is an area shielded so that light does not enter. Also, the VOB 902 is provided adjacent to the head (on the top side) of the aperture pixel area 903 in the vertical direction, and is an area shielded so that light does not enter. The aperture pixel area 903 and the optical black areas 901 and 902 have the same structure, and the aperture pixel area 903 is not shielded, whereas the optical black areas 901 and 902 are shielded. Hereinafter, the pixels in the optical black areas are called OB pixels. Normally, OB pixels are used to obtain a reference signal whose signal level is a reference, that is to say a black reference signal. The aperture pixels of the aperture pixel area 903 each accumulate an electric charge generated according to incident light, and output the electric charge.
FIG. 10 shows an example of a circuit of a unit pixel (corresponding to one pixel) in the CMOS image sensor. A photodiode (hereinafter, called a PD) 1001 receives an optical image formed by an imaging lens, generates an electric charge, and accumulates the electric charge. Reference numeral 1002 indicates a transfer switch that is configured by a MOS transistor. Reference numeral 1004 indicates a floating diffusion (hereinafter, called an FD). The electric charge accumulated by the PD 1001 is transferred to the FD 1004 via the transfer MOS transistor 1002, and then converted to a voltage and output from a source follower amplifier 1005. Reference numeral 1006 indicates a selection switch that collectively outputs one row-worth of pixel signals to a vertical output line 1007. Reference numeral 1003 indicates a reset switch that, with use of a power source VDD, resets the potential of the FD 1004, and resets the potential of the PD 1001 via the transfer MOS transistor 1002.
FIG. 11 is a block diagram showing an exemplary configuration of a CMOS image sensor. Note that although FIG. 11 shows a 3×3 pixel configuration, normally the number of pixels is high, such as several millions or several tens of millions. A vertical shift register 1101 outputs signals from row select lines Pres1, Ptx1, Psel1, and the like to a pixel area 1108. The pixel area 1108 has the configuration shown in FIG. 9, and has a plurality of pixel cells Pixel. Even-numbered columns and odd-numbered columns of the pixel cells Pixel output pixel signals to vertical signal lines of a CH1 and a CH2 respectively. A constant current source 1107 is connected as a load to the vertical signals lines. A readout circuit 1102 receives an input of a pixel signal from a vertical signal line, outputs the pixel signal to a differential amplifier 1105 via an n-channel MOS transistor 1103, and outputs a noise signal to the differential amplifier 1105 via an n-channel MOS transistor 1104. A horizontal shift register 1106 controls the switching on/off of the transistors 1103 and 1104, and the differential amplifier 1105 outputs a difference between the pixel signal and the noise signal. Note that although the output path configuration in FIG. 11 is a two-channel configuration including CH1 and CH2, high-speed processing is made possible by increasing the number of output paths. For example, if a total of eight output paths (in other words, four output paths both above and below in the image sensor configuration) are provided, eight pixels can be processed at the same time.
Using the differential amplifier described above enables obtaining an output signal from which noise unique to the CMOS image sensor has been removed. However, if there is variation between the characteristics of the output amplifiers of CH1 and CH2, a substantially uniform level difference occurs in each column. This is called vertical pattern noise.
Meanwhile, the pixels have a common power source and GND. If the power source and GND fluctuate during a readout operation, the pixels read out at that time have a substantially uniform level difference. Normally, readout is performed in an image sensor row-by-row, from left to right, beginning at the top left of the screen. The level difference occurring due to fluctuation of the power source and the GND appears as a different level difference for substantially each row. This is called horizontal pattern noise.
As described above, there is the problem that stripe noise occurs due to the structure of the CMOS image sensor, and this stripe noise tends to be more prominent as the specifications are improved. Since the vertical pattern noise is unique pattern noise determined by the characteristics of the output amplifiers, correction can be performed by correcting variations in each output amplifier. On the other hand, if the fluctuation of the power source and the GND is random, the horizontal pattern noise also becomes random.
As a technique for correcting such random pattern noise, Japanese Patent Laid-Open No. 7-67038 discloses a method of calculating a line average value for pixel signals of OB pixels, and subtracting the line average value from the pixel signals of aperture pixels in that row.
However, in an image that has a large amount of random noise, calculating a correction value for correcting stripe noise is difficult. This is pointed out in Japanese Patent Laid-Open No. 7-67038, Japanese Patent Laid-Open No. 2005-167918, and the like as well. According to Japanese Patent Laid-Open No. 2005-167918, if the stripe noise is reduced to from ⅛ to 1/10 of the random noise, the stripe noise becomes buried in the random noise, and thus becomes difficult to see. In view of this, Japanese Patent Laid-Open No. 2005-167918 discloses a method in which noise is mitigated by adding random noise.
However, with the correction method of subtracting the line average value of pixel signals of OB pixels, there are often cases in which the correction is insufficient, such as the case in which stripe noise occurs due to under-correction and over-correction. This phenomenon can often be seen in images containing a large amount of random noise, such as images captured at a high ISO speed. As the amount of random noise in an image rises, more OB pixels are necessary to obtain a proper correction value. Also, even given that the stripe noise is difficult to see if it is ⅛ to 1/10 or less of the random noise value as disclosed in the above-mentioned Japanese Patent Laid-Open No. 2005-167918, calculating a correction value such that the stripe noise becomes difficult to see necessitates approximately 400 or more OB pixels per row. However, allocating 400 columns or more to OB pixels in the layout of the CMOS image sensor cannot be said to be practical in view of the requirement for an increase in the number of pixels and high-speed imaging.